The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The present disclosure relates generally to an indicating system and a method for locating a downhole apparatus, and in particular, to a system and a method for locating a downhole apparatus inside a pressure containing system.
In the process of measuring and working on oil and gas wells, downhole tools and coiled tubing are frequently employed. During the use of the downhole tools, various depth measurement systems are employed to track the location of the tools with respect to the surface pressure containing equipment or to features in the well itself. Such surface pressure containing equipment is commonly referred to as a blow out preventer (BOP) stack, generally incorporating one or more BOPs, some length of riser (simple straight pipe), and one or more mechanisms to provide a seal between the BOP stack's pressure barrier and the conveyance method (e.g. coiled tubing, wireline, slickline, jointed pipe, and the like).
As an example, a key underlying measurement method for jointed pipe includes measuring the length of the pipes before they are put into the wellbore. For continuous conveyance methods, two means are generally employed: 1) measuring a rotation of the spool; and 2) pressing a friction wheel against the conveyance means and measuring a rotation to determine an amount of the conveyance means that has moved in or out of the well. Occasionally point markers are employed to indicate a specific location. Such markers include magnetized area, paint, plated on conductive areas, collars, and modifications to the cross section of the conveyance means (such as rolled in grooves or dents).
When a friction wheel is used as the sole means of locating the downhole tool with respect to an internal feature of the well and/or the BOP stack, it is often supplemented by references to a feature of the well such as the location of the casing collars, gamma ray patterns, nipple locations, or tagging the bottom. Any small errors (or permanent changes in the length of the conveyance, such as the increase in length seen in coiled tubing) may accumulate as a downhole tool is run in and out of the well, leading to uncertainty as to when the tool has been pulled back into the BOP stack for removal. This uncertainty can reach hundreds of feet; it is not unheard of to have operators be pulling out of hole at high speed, thinking they are still at significant depths, and then have their tool pulled into the sealing means. This often results in the failure of the joint between the tool and the conveyance means, leading to the dropping of the tool. This is sufficiently common in the wireline and slickline world that “tool traps” are commonly available that latch onto the tool when it is pulled up into them and prevent it from dropping even if the wire fails.
Another key challenge comes during pressure deployment; the tools must be very precisely located relative to the BOP rams so that the rams may be closed on a part of the tool that is prepared for this (commonly called a deployment bar). Missing the appropriate location by a few inches can mean major damage to the tool, dropping the tool, or a blowout. Reverse deployment (removing the tool) is complicated by the uncertainty in the location after the friction wheel errors have accumulated. This can be compensated by gently tagging the sealing mechanism to re-establish the relationship between the tool and the BOP stack.
It is always desirable to provide an indicating system and a method of locating a downhole apparatus disposed in a pressure cavity of a well.